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867 



TERTIARY FAUNA OF FLORIDA 



tion. In a single species the ligament, while quite separate from the pit, has 

 itself sunken below the dorsal shell margin, only its most anterior point re- 

 maining at the surface. {M. iristis Reeve, Conch. Icon.) 



Having indicated and named the characteristic portions of the hinge- 

 armature, it remains to say a few words as to the relative attitude of the 

 different parts. It must be borne in mind that the proliferations of the 

 mantle, which supply the exudations from which the shell -substance is crys- 

 tallized out, are delicate and filmy, unable to " stand alone." If, by some 

 accident, the beginning of the hinge or any part of it is abnormally shaped, 

 subsequent depositions must be laid down upon the abnormal basis and 

 correspondingly modified. In short, as soon as the shelly valves are formed 

 they represent, in relation to the soft parts they enclose, an extraneous rigid 

 mold or body composed of two parts which react dynamically upon each 

 other through the intermediation of the soft parts contained between them. 

 The initiatory form of the shell is as purely genetic as any portion of the 

 animal can be; the subsequent development must be largely guided by it. 

 Mutations foreign to this plan can only be brought about by environmental 

 forces still more energetic. 



The distribution of the parts of the hinge in single species is remarkably 

 uniform, but if groups of species are considered, the types are seen to gradu- 

 ally approach and almost merge, one in another. Sharp generic distinctions 

 can seldom be drawn, and there are many groups named, sectionally or even 

 generically, which owe their verbal distinctness to wilful or unconscious 

 ignoring of the details of structure in other parts of the family. 



The lateral laminje were originally determined from the umbones as a 

 focus, but this was an ancient event and, for practical purposes, is much 

 obscured in the existing conditions of the hinge. The distance to which 

 they extend on either side of the beaks, and their greater or less continuity 

 between the beaks and the distal portions of the teeth, are variable dynamic 

 functions depending upon the form of the dorsal arch of the valves and the 

 strains to which the valves are subjected in the station preferred by each 

 particular species. In general, the geologically more ancient forms have the 

 lateral laminae more adjacent than in their modern descendants, other things 

 being equal, which is what might be expected theoretically. Taking the 

 different species in one contemporaneous group, much variation may be found 

 in the distance of the laterals from the beak, as well as in their form, but 

 these features do not seem to possess any very great systematic importance. 



